This invention is generally in the field of security techniques, and relates to a security system for encoding documents and other valuable items with code patterns, and a method for reading these code patterns.
It is often the case that documents or other valuable items need to be protected from tampering, falsification and unauthorized use. The accepted way of protection consists of introducing one or more security means into, or attaching these means to, a document or an item. The documents and items to be protected include ID cards, passports, licenses, security passes, currency, checks, travel tickets, keys and key cards, and the like. The most widely used encoded security means are so-called xe2x80x9coptical bar codesxe2x80x9d and magnetic strips. Such encoded security means may be either visible or hidden from view.
Conventional optical bar codes suffer from the drawback associated with the fact that dust or dirt incidentally appearing on either a data recording medium or a data reader device may cause read errors. Additionally, in magnetic strips, the recorded data may be damaged by the influence of an ambient magnetic field or an elevated temperature.
U.S. Pat. No. 4,883,949 discloses a system with increased code reliability. In this system, data recording means are defined by intermittent patterns of different materials that can affect the high frequency impedance of the magnetic coil of a reading head. The pattern materials may differ in electric conductivity and/or magnetic permeability.
Another common drawback of the optical bar codes and magnetic strips, is that they can be easily read out and duplicated by conventional means. To increase the security of important documents and valuable items from unauthorized use or falsification, it was proposed to incorporate a magnetic bar code strip or other pattern hidden within the document or item substrate or concealed under other xe2x80x9cdummyxe2x80x9d security elements such as optical bar codes or holograms.
In the known devices of the kind specified, magnetic materials were used mostly in the form of magnetic inks containing powders of high magnetic coercivity, e.g., Fe2O3, BaFe12O19 and the like. However, it is recognized that the strong persisting magnetic field of such materials allows for reading out and duplicating the encoded pattern.
In U.S. Pat. No. 5,616,911, it is proposed that the hidden magnetic bar code pattern be formed using the low concentration magnetic ink exhibiting very low, approximately zero, remanance. Such magnetic inks may be formed from soft magnetic materials including Fe-powder, ferrites, Sendust alloy powder, amorphous alloys in a powder form and the like. The concentration of these materials in an ink or binider medium of less than 10% by volume is preferred. A reading head utilizing an in-field magneto-resistive sensing element is employed to magnetize the bar code pattern bands and detect their width or the space between them, whilst they move with respect to the head. After being read, the code pattern exhibits practically no residual magnetic field and is not readable by a magnetic viewer. One of the disadvantages of the proposed security system is in that the applied soft magnetic powders are easily available and their magnetic signature is not specific, and therefore the document counterfeiting is still possible. Another disadvantage is in that the sensitive area of the reading head with a magneto-resistor sensing element is large. The size of the code pattern elements and the distance between them cannot be smaller than the reading head sensitive area, otherwise these elements would not be unambiguous. Therefore, the information density of such a coding system is intrinsically low. Moreover, the reading head resolution deteriorates strongly with the increase in the distance between the magneto-resistive sensing element reading head and the code pattern. The same is relevant for the bar code system disclosed in the above-indicated U.S. Pat. No. 4,883,949.
U.S. Pat. No. 5,801,630 discloses a method for preparing a magnetic material with a highly specific magnetic signature, namely with a magnetic hysteresis loop having large Barkhausen discontinuity at low coercivity values. This material is prepared from a negative-magnetostrictive metal alloy by casting an amorphous metal wire, processing the wire to form longitudinal compressive stress in the wire, and annealing the processed wire to relieve some of the longitudinal compressive stress. One disadvantage of such material in security pattern applications is the relatively large wire diameter, which is approximately 50 xcexcm. Another disadvantage is the complicated multi-stage process of the wire preparation. Yet another disadvantage of the material is the amorphous wire brittleness which appears due to wire annealing Such brittleness will prevent using the material in security patterns formed in paper documents like currency, checks, passports, etc.
According to another known technique, a glass-coated magnetic microwire is used as a magnetic material having unique magnetic properties. This microwire is cast directly from the melt by a modified Taylor method, which is well known in the art, and is disclosed, for example, in the article of I. W. Donald and B. L. Metcalf xe2x80x9cThe Preparation Properties and Applications of Some Glass-Coated Metal Filaments Prepared by the Taylor-Wire Processxe2x80x9d, Journal of Materials Science, Vol. 31, pp 1139-1149, 1996. It is an important feature of the Taylor process that it enables pure metals and alloys to be produced in the form of a microwire in a single operation, thus offering an intrinsically inexpensive method for the manufacture of microwire. The glass-coated microwires can be produced with very small diameters (ranging from 1 xcexcm to several tens micrometers), from a variety of magnetic and non-magnetic alloys and pure metals. Magnetic glass-coated microwires may be prepared also with amorphous metal structures, as disclosed in the article of H. Wiesner and J. Schneider xe2x80x9cMagnetic properties of Amorphous Fe-P Alloys Containing Ga, Ge and Asxe2x80x9d, Physica Status Solidi, Vol. 26, pp 71-75, 1974, and other publications cited as references in the above-indicated review of Donald and Metcalf. These amorphous magnetic glass-coated microwires have good mechanical strength, flexibility and corrosion resistance, so that they can be easily incorporated in paper, plastic and other document substrate materials, Amorphous magnetic glass-coated microwires are characterized by their unique response which may resemble, inter alia, that of die-drawn amorphous wires of the above cited US ""630.
There is accordingly a need in the art to improve security systems by providing a novel security system utilizing a code pattern formed by magnetic elements with extremely low coercivity and high permeability, and a novel reading head and method for reading the code pattern.
It is a major feature of the present invention to provide such a system that has a high density magnetic read-only code pattern, which is not visible to the Individual""s eyes, and a reading head capable of reading this code pattern
It is another feature of the present invention to provide such a reading head that is designed to match the unique response characteristic of the code pattern material.
The main idea of the present invention is based on the following. A read-only code pattern, to be attached to a document that is to be protected, is formed by magnetic elements having extremely low coercivity (substantially less than 10 xcexcm) and high permeability (substantially higher than 20000). A reading head, in addition to the conventionally used magnetic sensing element (e.g., coils), is provided with a magnetic means (e.g., at least two permanent magnets) creating a static magnetic field of a specific configuration. This static magnetic field, on the one hand, affects the magnetic elements so as to provide their magnetic response to this static filed, and, on the other hand, has such a configuration as to define an extended narrow region (plane) where the static magnetic field vector is substantially equal to zero. The sensing element is located substantially within his zero-field plane. In other words, the magnetic field produced by the magnetic means has a high gradient in the sensitivity zone of the sensing element. Due to the above magnetic characteristics of the magnetic elements, they will produce an effective (noticeable by the sensing element) response only within this narrow (i.e., plane-like) zone, while being saturated by the static magnetic field at any location out of this plane. The magnetic response of these magnetic elements would not be readable with sufficient resolution by the sensing element only, without the use of the high-gradient static magnetic field. Moreover, to read the code pattern, the magnetic elements should be displaced with respect to the sensing element and located outside the magnetic means, provided during this displacement each of the magnetic elements passes the zero-field plane in a reading direction perpendicular to this plane.
The pattern may be representative of a binary code, wherein binary xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d are defined either by different distances between the adjacent magnetic elements, or by different parameters of the magnetic elements (e.g., materials or dimensions).
There is thus provided according to one aspect of the present invention a security system for protecting an item by forming it with a predetermined code pattern, the system comprising:
(a) a plurality of spaced-apart magnetic elements forming said code pattern, wherein said magnetic elements are made of at least one magnetic material having a coercivity substantially less than 10 xcexcm and a permeability substantially higher than 20000;
(b) a reading head comprising a sensing element of a kind responsive to signals generated by the magnetic elements, and a magnetic means of a kind producing a high gradient static magnetic field and defining an extended narrow region where the static magnetic field vector is substantially equal to zero, said sensing element being mounted with respect to the magnetic means so as to be located substantially within said region to detect the signals produced by the magnetic elements when they pass said extended narrow region in a reading direction perpendicular to said region; and
(c) a detector detecting the response of said sensing element and generating data representative thereof
The term xe2x80x9cHexended narrow regionxe2x80x9d used herein signifies a plane-like zone. Generally speaking, the zero-field extended narrow region is located within a plane perpendicular to a reading direction in which the magnetic elements are to be displaced with respect to the reading head. Preferably, the zero-field region, a sensing area, is less than 50 xcexcm in the axis along the reading direction.
The sensing element may be a conventional magnetic head, e.g., a coil wound onto a core, or a magnetoresistor element. The magnetic means producing the high gradient static magnetic field may include at least two spaced-apart permanent magnets. The magnets are oriented with respect to each other such that their axes of magnetization are aligned in a spaced-apart parallel relationship, and each pole of one permanent magnet faces an opposite pole of the other permanent magnet.
The magnetic elements are, preferably, glass-coated microwires, which may be produced in a one-stage casting process from an alloy having substantially zero magnetostriction. Preferably, the alloy is cobalt based, including more than 70% cobalt by atomic percent.
The microwires have very small diameters, so that they can be embedded into the document substrate without causing noticeable changes in the document thickness or form. The position of the microwires in the document cannot be detected visually or by touch. The code pattern formed by microwires is not visible with the magnetic viewer, owing to the fact that the microwire is a soft magnetic material and demonstrates a very low remanence. The microwire pattern density is considerably higher than that obtained with the known magnetic materials like magnetic inks. The microwire code pattern cannot be read with known readers, including those which detect soft magnetic materials, since the sensing area of known readers is much larger than the distance between the microwire pieces in the code pattern of the present invention. The microwire code pattern can be read only and solely with a specific reader device (reading head) according to the present invention. Hence, the microwire code pattern increases substantially the document security.
The code pattern may be attached directly to the item, or to a separate label attached to the item.
According to another aspect of the invention, there is provided a reading head for reading an intermittent code pattern formed of a plurality of spaced-apart magnetic elements made of at least one magnetic material, when the magnetic elements are displaced in a reading direction with respect to the reading head, the reading head comprising:
a magnetic means producing a high gradient static magnetic field defining an extended narrow region where the static magnetic field vector is substantially equal to zero; and
a sensing element mounted with respect to the magnetic means so as to be located substantially within the zero-field region, the sensing element being thereby responsive to signals generated by the magnetic elements, when each of the magnetic elements, while being displaced in said reading direction, is located in said zero-field region.
According to yet another aspect of the present invention, there is provided a method for reading a code pattern formed by a plurality of spaced-apart magnetic elements made of at least one magnetic material having a coercivity substantially less tan 10 A/m and a permeability substantially higher than 20000, the method comprising the steps of:
(i) creating a static magnetic field with substantially high gradient, such that the static magnetic field vector is substantially equal to zero within an extended narrow region;
(ii) providing a sensing element of a kind responsive to signals generated by the magnetic elements and locating said sensing element substantially wit the zero-field region;
(iii) providing relative displacing of the code pattern relative to said sensing element in a reading direction perpendicular to said extended narrow region, such that the code pattern passes through said static magnetic field, thereby exciting each of the magnetic elements to generate the signals whilst passing the zero-field region; and
(iv) detecting the response of the sensing element to said signals generated by the magnetic elements.